Fiber-reinforced composite materials using carbon fibers and aramid fibers as reinforced fibers are widely utilized for structural materials of air planes and automobiles and for sporting uses such as tennis rackets, golf shafts and fishing rods and for general industrial uses by taking advantage of their high specific strength and specific elastic modulus. Employed methods for producing fiber-reinforce composite materials include: the method in which a prepreg, a sheet-shaped in-process material, impregnating reinforced fibers with an uncured matrix resin is prepared, and the prepregs are laminated multiply and thermally cured; and the resin transfer molding method in which a liquid resin is poured in the reinforced fibers disposed in a mold and thermally cured. Among these methods, the method using the prepreg has an advantage that the fiber-reinforced composite material having high performance is easily obtained because an orientation of the reinforced fibers can be strictly controlled and a design flexibility for a lamination structure is high. As a matrix resin used for this prepreg, thermosetting resins are mainly used in terms of heat resistance and productivity, and especially, epoxy resins are suitably used in terms of dynamic properties such as adhesiveness to the reinforced fibers.
In recent years, in addition to a trend toward weight lightening by substituting the fiber-reinforced composite materials for conventional materials such as metals, the trend toward further weight lightening of the fiber-reinforced composite material itself has been recently activated in various uses. In associated with that, it has become more popular to widely use the fiber-reinforced composite materials employing the reinforced fibers with higher elastic modulus. This allows the material to be thinned and lightened with keeping a rigidity of the material as it is. In this regard, however, when the reinforced fiber having the higher elastic modulus is used, a strength property such as fiber direction compression strength conversely tends to be reduced.
To improve the strength property such as fiber direction compressive strength (static strength property), it is effective that the elastic modulus of the matrix resin is enhanced. A combination of an amine-type epoxy resin component is an effective procedure that can enhance the elastic modulus while a harmful effect on the heat resistance and toughness of a cured matter is minimized. However, it has been a problem that impact strength is scarcely improved even in this case. For example, Patent Document 1 (JP Sho-62-1717-A) discloses that by combining the amine-type epoxy resin having the high elastic modulus, fiber direction bending strength and interlayer shearing strength, which are strongly correlated with the fiber direction compressive strength, were remarkably improved, but impact resistance was not sufficiently enhanced. Patent Document 2 (JP 2004-269600-A) discloses that a tubular body was obtained by combining carbon fibers having the high elastic modulus of which tensile elastic modulus is 375 GPa with the epoxy resin such as amine-type epoxy having the high elastic modulus, and it exerted high torsional strength, but the impact resistance was still insufficient.
In this regard, in order to increase the impact resistance of the fiber-reinforced composite material, which is composed of the reinforced fibers and the matrix, it is necessary to enhance the extension degree of the reinforced fiber and the extension degree and the toughness of the matrix resin. Among them, in particular, it has been described to be important and effective that the toughness of the matrix resin is enhanced, and it has been attempted to modify the epoxy resin.
Conventionally, as for methods to improve the toughness of an epoxy resin, some methods such as combining a rubber component or combining a thermoplastic resin have been attempted. However, these methods have had problems: deterioration of a process property due to reduction of the heat resistance and an increased viscosity; and quality reduction such as occurrence of voids.
Also, the method in which a fine phase separation structure is stably formed during curing the epoxy resin to largely enhance the toughness of the cured epoxy resin by adding a copolymer composed of styrene-butadiene-methyl methacrylate or a block copolymer such as a block copolymer composed of butadiene-methyl methacrylate has been proposed (Patent Document 3 and 4 [see JP 2003-535181, International Publication 2006/077153 Pamphlet]). However, because of excessively high crosslinking density in such a composition, plastic deformation capacity was insufficient, improvement for the enhancement by combining the block copolymer was not observed, and particularly when the reinforced fiber having the high elastic modulus was applied, the dynamic property of the resulting fiber-reinforced composite material was not sufficient. When dicyandiamide, which is a curing agent being suitable for prepreg molding that the fiber-reinforced composite material with high performance is easily obtained and giving the high plastic deformation capacity, was used in combination with the amine-type epoxy resin, the combined block copolymer formed a rough and large phase separation structure and the dynamic property tended not to be improved.    Patent Document 1: JP Sho-62-1717-A    Patent Document 2: JP 2004-269600-A    Patent Document 3: JP 2003-535181-A    Patent Document 4: International Publication 2006/077153 Pamphlet